Media transmission is affected by information loss due to transmission errors in many transmission channels without Quality of Service (QoS).
One example of a media stream is a video bit stream. Due to the spatial and temporal dependency structure of modern video codecs like H.264/AVC or especially the scalable extension of H.264/AVC (SVC) [A1], the effect of losses on the decoded video quality mainly depends on which parts of the bit stream are affected.
In unidirectional channels forward error correction (FEC) technologies can be used to combat this losses. Many protection schemes have been proposed addressing this issue like unequal error protection (UEP) [A5] or priority encoding transmission (PET) [B3]. Both approaches give more protection to lower and more important layers. But first in [A2] it has been recognized, that generating protection over all depending layers also redundancy symbols of upper layers can help increasing protection for lower layers. This can be very beneficial in transmission schemes where multiple layers of a scalable video stream are transmitted simultaneous like e.g. layered multicast [A6].
Various dependency structures can be considered. One particular important dependency structure is introduced by motion compensation, where a reference picture is used to predict another picture. If a referenced picture or a slice of that reference picture is lost, pictures and slices that are referencing it are affected too. Therefore, in a sequence of pictures that reference each other, the first pictures in the chain are most important. A loss at some point typically affects all succeeding pictures in some way. Hence, it is desirable to protect as many pictures as possible in such a dependency chain.
Another set of dependency structures is introduced by SVC, where a base-layer is referenced by an enhancement layer. This enhancement layer can be further referenced by another enhancement layer and so on. Let l be the identifier of a layer out of 0 to L-1, where l=0 is the base layer, l=1 identifies the first enhancement layer referencing the base layer. In SVC, the loss of any layer x leads to unusable decoding results for all layers l>x referencing x. Therefore, priority must be given to correctly obtaining any layer<l when a certain quality is requested, represented by a particular value of 1.
Forward error correction (FEC) can be used to enhance reliability of media transmission over non-QoS channels. Independent FEC protection as used by nowadays schemes of an enhancement layers does not take the relation of layers into account. In such a scheme, typically for a number of k source symbols, a number of p=n−k redundancy symbols are generated. Let us assume the usage of a maximum distance separable (MDS) FEC code, so that each number of erasures of symbols smaller or equal to p can be overcome. Even, when using stronger FEC protection (more redundancy symbols) for the more important lower layers (this is also known as unequal error protection), there is no guarantee, that in any error condition the reconstruction of relatively stronger protected layers is of higher probability than for less strong protected layers.
If a lower layer is lost, higher enhancement layers cannot be decoded due to missing references as depicted in FIG. 19. In this example layer 1 at time t is lost due to transmission error (“error”), so that layers with l>1 can not be decoded. Successfully received FEC protection of enhancement layers is useless.